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EW CATARACT 32 May 2015 by Lauren Lipuma EyeWorld Staff Writer matism in with-the-rule patients and a higher amount in against-the-rule patients and use the age factor to set the refractive target. Putting it all together Dr. Yeu summarized these pearls with 2 formulas for determining the total amount of astigmatism correc- tion needed when planning surgery. For with-the-rule patients, start with the amount of anterior corneal astigmatism, add the amount of SIA, subtract half a diopter to account for the posterior corneal astigmatism, and then add the age factor—add 0.25 D for patients under 65, or 0 D for patients over 65. For against-the- rule patients, start with the anterior corneal astigmatism, subtract the SIA, add 0.3 D of posterior corneal astigmatism, and then add the age factor, she recommended. EW References 1. Koch DD, Ali SF, Weikert MP, Shirayama M, Jenkins R, Wang L. Contribution of posterior corneal astigmatism to total corneal astig- matism. J Cataract Refract Surg. 2012 Dec; 38(12): 2080–7. 2. Hayashi K, Hirata A, Manabe S, Hayashi H. Long-term change in corneal astigmatism after sutureless cataract surgery. Am J Ophthalmol. 2011 May;151(5):858–65. Editors' note: Dr. Yeu has no financial interests related to this article. Contact information Yeu: eyeulin@gmail.com She recommends including the potential for this shift in surgical planning as an "age factor" when setting the refractive astigmatic target. For patients under 65 years of age, Dr. Yeu sets a refractive target of between 0.25 and 0.4 D of residual with-the-rule astigmatism, and in patients over 65, she sets a target of either no or minimal residual astigmatism. Pearl 5: Oblique astigmatism is likely with-the-rule astigmatism that is marching against-the- rule When treating oblique astigmatism, you can treat "spot on" based on the anterior corneal astigmatism, Dr. Yeu said, but oblique astigmatism tends to be with-the-rule that is moving against-the-rule over time, so con- sider choosing a higher power toric lens in those cases. Pearl 6: Effective lens position can affect the true toricity correction of the lens Toricity correction is less predict- able in long and short eyes, due to the methods used for measuring the effective lens position. To avoid refractive surprises, in shorter eyes consider decreasing the toric power beyond the nomogram recommen- dation, and in longer eyes consider increasing the toric power, Dr. Yeu said. When effective lens position values are inconsistent, consider correcting a lower amount of astig- demonstrated that 84% of patients have with-the-rule posterior corneal astigmatism, so it is important to include the posterior cornea when calculating the total corneal astig- matism, Dr. Yeu said. Posterior corneal astigmatism functions like a negative lens, con- tributing to the total ocular against- the-rule astigmatism when it is aligned with-the-rule. Generally, the greater the amount of anterior with- the-rule astigmatism the patient has, the greater the amount of posterior with-the-rule astigmatism the pa- tient will have as well, Dr. Yeu said. On average, patients who have an- terior with-the-rule astigmatism will have 0.5 D of posterior with-the-rule astigmatism, and those who have anterior against-the-rule astigmatism will have 0.3 D of with-the-rule pos- terior astigmatism. Unlike anterior astigmatism, posterior astigmatism shows no drift over time. Current corneal topographers do not consistently measure poste- rior corneal astigmatism well, Dr. Yeu said, but there are promising software upgrades to 2 systems—the Cassini and the Galilei (Ziemer, Port, Switzerland)—that could capture it more accurately in the future. In- traoperative aberrometry, however, does measure posterior corneal astig- matism, so it may offer a solution to this problem. Pearl 3: Understand the astigmatic effects of corneal incisions Do not underestimate the impor- tance of the astigmatic effects of your own corneal incisions, Dr. Yeu said. To be a refractive cataract sur- geon, you have to refine your own surgically induced astigmatism (SIA) by comparing your preoperative and postoperative topography measure- ments. Ideally, preop measurements should be compared with topogra- phy measurements at 1 or 2 months postop, she said. Pearl 4: Consider the patient's age when setting the astigmatic target A 2011 study in the American Jour- nal of Ophthalmology showed that against-the-rule astigmatic drift tends to occur with age—on average, patients experience a 0.37 D shift over 10 years, Dr. Yeu said. Factor in posterior corneal astigmatism, age, and effective lens position when setting a refractive astigmatic target, physician recommends C orrecting astigmatism during cataract surgery requires careful preopera- tive planning. In addition to the anterior corneal curvature, several factors can have a dramatic impact on the refractive outcome if not properly accounted for, said Elizabeth Yeu, MD, assis- tant professor, Eastern Virginia Med- ical School, and in private practice, Virginia Eye Consultants, Norfolk, Va. Speaking at the 2014 Combined Ophthalmic Symposium, Dr. Yeu described the factors that surgeons should take note of, and offered 6 pearls for successful management of astigmatism when planning cataract surgery. Pearl 1: Obtain an accurate assessment of anterior corneal astigmatism Accurately assess the anterior cor- neal astigmatism in both axis and magnitude, Dr. Yeu said. She recom- mended obtaining a minimum of 2 reliable measurements of anterior corneal curvature from 2 different methods, such as manual keratom- etry, optical biometry, and corneal topography. When it comes to corneal topography, Placido disc-based measurements offer an instanta- neous and accurate assessment of the corneal surface, but the accuracy of those measurements are depen- dent upon a good ocular surface, Dr. Yeu said. Having a dry ocular surface can yield flawed results, so altern - tive topography methods may be best for dry eye patients. One option is the Cassini corneal topographer (i-Optics, The Hague, the Nether- lands), which uses ray tracing tech- nology that makes it inherently less dependent on ocular surface health. Pearl 2: The posterior cornea plays a significant role in total refractive astigmatism A 2012 study published in the Jour- nal of Cataract & Refractive Surgery Six preop pearls for astigmatism management Dual Scheimpflug image sh wing –0.65 D of astigmatism on the posterior cornea. Posterior corneal astigmatism functions like a negative lens, contributing to the total ocular against-the-rule astigmatism when it is aligned with-the-rule. Source: Douglas Koch, MD, and Wang Li, MD